Hydrogenated graphene with surface doping and bandgap tunability

ABSTRACT

A graphene compound made from the method of preparing graphene flakes or chemical vapor deposition grown graphene films on a SiO 2 /Si substrate; exposing the graphene flakes or the chemical vapor deposition grown graphene film to hydrogen plasma; performing hydrogenation of the graphene; wherein the hydrogenated graphene has a majority carrier type; creating a bandgap from the hydrogenation of the graphene; applying an electric field to the hydrogenated graphene; and tuning the bandgap.

REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefits of U.S. ProvisionalPatent Application 61/701,029 filed on Sep. 14, 2012, and U.S. patentapplication Ser. No. 13/942,257 filed on Jul. 15, 2013, the entirety ofeach is herein incorporated by reference.

BACKGROUND

This disclosure provides a method for introducing a bandgap in singlelayer graphite (graphene) on a SiO₂ substrate, while also allowing forindependent control of the majority carrier type via surface adsorbates.Specific applications of the invention include patterning graphenesamples for nanocircuit design and device integration at various scales,for example, p-n junctions. The technique is reversible, such that thedopant atoms introduced into the graphene can be removed whilepreserving the original graphene band structure.

SUMMARY OF DISCLOSURE

The addition of hydrogen to the 2-dimensional material graphene resultsin considerable changes to the electrical properties of the film, namelythe conversion from a semi-metallic behavior to a semi-insulatingbehavior. We have demonstrated the temperature dependence of theresistivity of chemical vapor deposition (CVD) grown graphene uponhydrogenation in a plasma enhanced chemical vapor deposition (PECVD)reactor, where we have electrically observed a bandgap opening inhydrogenated graphene, and have shown that at 0 V_(g) the bandgap ishigher for higher hydrogen to carbon (H/C) ratios.

Additionally, we have demonstrated that hydrogenated graphene on a SiO₂substrate is an n-type material when it is free of adsorbates.Furthermore, we have demonstrated the ability to tune the bandgapopening.

And, we have demonstrated the capability to convert the majority chargecarrier from electrons to holes using surface adsorbates such as water.Still furthermore, we have demonstrated that in the temperature regime220 K-375 K the bandgap of hydrogenated graphene has a maximum value atthe charge neutrality point (CNP), is tunable with an electric fieldeffect, and is higher for higher H/C ratios.

DESCRIPTION OF THE DRAWINGS

The following description and drawings set forth certain illustrativeimplementations of the disclosure in detail, which are indicative ofseveral exemplary ways in which the various principles of the disclosuremay be carried out. The illustrated examples, however, are notexhaustive of the many possible embodiments of the disclosure. Otherobjects, advantages and novel features of the disclosure will be setforth in the following detailed description when considered inconjunction with the drawings.

FIG. 1 illustrates Shift of the CNP and maximum resistivity (r_(max))with surface doping. (a) Optical image of a typical graphene device andelectrical schematic. (b) r at 296 K as a function of V_(g) for graphenesample G-X1 (black trace) and three separate levels of hydrogenation(red, green, and blue traces) with water adsorbates. The solid curvesare Lorentzian fits to the data. The dashed line is a guide to the eye.Purple trace: r versus V_(g) at 296 K for the D/G of 2.94 after heatingto 375 K (HG-X1). The shift of the CNP to negative V_(g) demonstratesthe conversion of the majority carrier type from p to n. (c) r versusV_(g) at 295K for graphene sample G-Hb (black trace) and two separatelevels of hydrogenation (red and blue curves) without water adsorbates.(d) Relative work functions of the graphene (G) (center), SiO₂ surface(S), hydrogenated graphene (HG) (top), and hydrogenated graphene withwater adsorbates (HG+W) (bottom).

FIG. 2 illustrates Electrical properties of n-doped hydrogenatedgraphene. (a) Raman spectra of the graphene device (square device) inFIGS. 2b-d before (G-Sq) and after hydrogenation (HG-Sq). The spectrahave been normalized to the G-mode intensity. (b) r as a function ofV_(g) at 296 K for G-Sq before heating to 375 K (black trace) and afterheating (red trace). The solid curves are Lorentzian fits to the data.Inset: temperature dependence of graphene demonstrates characteristicsemi-metallic properties. (c) r versus V_(g) from 50 K to 375 K for thehydrogenated device HG-Sq with a D/G ratio of 3.9. Data was taken afterpumping the chamber for 17 hours. Note that the CNP is located atnegative V_(g), indicating an n-type material. The solid lines areLorentzian fits to the data. The graphene (black trace) was measured atroom temperature. (d) r versus temperature for several different gatevoltages. The solid lines are fits to the VRH theory. Inset:Characteristic exponents T_(o), extracted from a fit to the VRH theory,versus V_(g). The black trace is a Lorentzian fit to the data.

FIG. 3 illustrates Electrical properties of p-type hydrogenated graphenebefore conversion to n-type. (a) Raman spectra of the graphene device(cross device) in FIG. 3b before (G-X2) and after hydrogenation (HG-X2).The spectra have been normalized to the G-mode intensity. (b) r versusV_(g) from 220 K to 375 K for the hydrogenated device HG-X2 with a D/Gratio of 2.1. Data was taken after pumping the chamber for 17 hours.Note that the CNP is to the left of the pristine graphene CNP, but tothe right of 0 V_(g). It is possible that this is due to unintentionaldoping of the initial graphene (CNP located at ˜30V). The solid linesare Lorentzian fits to the data. The graphene (black trace) was measuredat room temperature.

FIG. 4 illustrates Bandgap opening in hydrogenated graphene. (a)−Ln(1/r) versus ½KBT for the high temperature data in FIG. 2d . Thebandgap for a homogenous semiconductor is the slope of the curve. (b)Bandgap De versus V_(g) for the two different hydrogenated devices withD/G ratios of 3.9 (HG-Sq) and 2.1 (HG-X2). Solid curves are Lorentzianfits to the data.

DETAILED DESCRIPTION OF THE INVENTION

We report the first observation of the n-type nature of hydrogenatedgraphene on SiO₂ and demonstrate the conversion of the majority carriertype from electrons to holes using surface doping. Density functionalcalculations indicate that the carrier type reversal is directly relatedto the magnitude of the hydrogenated graphene's work function relativeto the substrate, which decreases when adsorbates such as water arepresent. Additionally, we show by temperature dependent electronictransport measurements that hydrogenating graphene induces a bandgap,and that in the moderate temperature regime [220 K-375 K], the bandgaphas a maximum value at the charge neutrality point (CNP), is tunablewith an electric field effect, and is higher for higher hydrogencoverage. The ability to control the majority charge carrier inhydrogenated graphene, in addition to opening a bandgap, suggestspotential for chemically modified graphene p-n junctions.

Exfoliated graphene flakes and CVD grown graphene films were prepared ona SiO₂ (275 nm)/Si (n-type arsenic doped) substrate, followed by thedeposition of Cr (10 nm)/Au (50 nm) contact electrodes. Hydrogenation ofthe graphene was performed according to the following conditions: 15-30W, 1.5 Torr H₂, 100 sccm H₂, 32° C., for 15-30 seconds. Thehydrogenation conditions (time and power) varied depending upon thedesired level of hydrogenation.

Raman spectroscopy was used to determine the relative defect densitiesin the films by a ratio of the G-mode intensity (1588 cm⁻¹, E_(2g)phonon mode) and the D-mode intensity (1345 cm⁻¹, appearing due tosymmetry breaking at defect sites). The D/G ratio is related to thedefect-free domain size of graphitic materials, in this case caused byhydrogen addition to the graphene sheet. With the PECVD conditionsstated, the resulting D/G ratios demonstrated saturation for eachpower/time and were repeatable for all devices. Raman spectra werecollected using a Renishaw MicroRaman Spectrometer with a 514 nm laserexcitation.

Tunability of the majority carrier type via surface doping removes therequirement for multiple gate electrodes for independent carrier typecontrol, avoiding the need for high quality dielectrics that aredifficult to grow on graphene and are susceptible to leakage currents.The additional capability of being able to introduce a bandgap ingraphene materials, in parallel to controlling the majority carriertype, makes hydrogenated graphene a promising method for nanocircuitdesign in a graphene-based system.

Graphene lacks a bandgap in its electronic spectrum, thus graphene'sconductivity cannot be turned off electronically as in conventionalsemiconductor materials. This hinders this unique material fromreplacing silicon-based electronics in logic operations. The absence ofa bandgap is one of the biggest hurdles that must be overcome beforegraphene can be used as an electronic material for use in logicoperations, and one that has sparked an intense research effort to thiseffect. Chemical functionalization of graphene is a promising method fortuning the material's unique band structure and majority carrier typefor future electronic and optical applications.

Graphane, a recently discovered completely hydrogenated derivative ofgraphene, is a stable two dimensional structure in which the sp² C—Cdouble bonds are hybridized to sp³ C—C single bonds by the addition ofhydrogen to the carbon lattice. Unlike graphene, which is a zero bandgapsemimetal, hydrogenated exfoliated and CVD grown graphene have beenshown to exhibit a strong temperature dependence (ΔR/ΔT<0)characteristic of semiconducting materials. Theoretically, the bandgapof hydrogenated graphene has been shown to depend upon the amount of Hcoverage on each side of the graphene film as well as the distributionand ordering of H atoms on the film, reaching values as high as 5.4 eV.Several recent experimental studies have shown that hydrogenatedgraphene has a bandgap. Haberer et al. have used angle-resolvedphotoemission spectroscopy (ARPES) to measure a bandgap inquasi-free-standing hydrogenated graphene on Au, where the size of thegap is tuned by varying the H/C ratio. Additionally, Balog et al. usingARPES have shown that hydrogen adsorbed onto the Moire superlatticepositions of graphene grown on an Ir(111) substrate also induces anappreciable bandgap, which is tunable by varying the H/C ratio as well.In spite of these significant achievements, many open questions stillremained as concerned graphene.

Here we show that hydrogenated graphene on SiO₂ is an n-type materialand electrically demonstrate the ability to tune the bandgap opening. Wereport on the ability to convert the majority carrier type fromelectrons to holes using surface adsorbates such as water, whichaccording to our density-functional theory (DFT) model is a consequenceof shifting of the material's work function relative to the substrate'swork function upon hydrogenation and subsequent adsorption/desorption ofatmospheric water.

Furthermore, we demonstrate that in the temperature regime 220 K-375 Kthe bandgap has a maximum value at the charge neutrality point (CNP), istunable with an electric field effect, and is higher for higher H/Cratios. This is the first report, to our knowledge, of the n-type natureof hydrogenated graphene on a SiO₂ substrate as well as the firstdemonstration of the complete reversibility of the majority carrier typewith surface doping. The temperature dependent resistivity ofhydrogenated graphene shows semiconducting behavior and is welldescribed by the variable-range hopping model. We show that in ourdevices a bandgap of up to 50 meV emerges at the CNP, and that the sizeof the gap can be tuned by varying V_(g) and/or the hydrogen coverage.

FIG. 1(a) is an optical micrograph of a typical graphene device andelectrical schematic used in our experiments. A Van der Pauw cross(referred to here as device X1) with arm dimensions of 500 nm and 200 nm(length and width respectively) which was characterized before (G-X1)and after hydrogenation (HG-X1). FIG. 1(b) shows ρ versus V_(g) at 295 Kfor HG-X 1+W (+W for “with adsorbed water”) and illustrates both theincrease in ρ and the shift in the CNP away from the pristine graphenestate with increasing levels of hydrogenation, comparable to previouslyreported hydrogenated graphene studies. The data shown in FIG. 1(b)(with the exception of the purple trace) was intentionally taken asquickly as possible with each experiment commencing within 10 minutes ofinitiation of chamber evacuation (P_(o)>1.0×10⁻⁴ Torr), thus notallowing for complete removal of physisorbed water. Ambipolar behavioris still observed even for our highest D/G ratios, though ρ at the CNPis seen to increase by a factor of 11, and the CNP has shifted to theright nearly 30V. The shift of the CNP to the right, indicating a largerfraction of p-type carriers, is attributed to atmospheric water adsorbedto the hydrogenated graphene surface. Such behavior indicates that thehydrogenated graphene is doped with holes while water is adsorbed to thesurface, and that the dopant level can be varied with different degreesof hydrogenation and/or surface water concentrations.

To remove the adsorbed water the sample was heated while continuouslymaintaining a vacuum (P<1.0×10⁻⁶ Torr). The purple trace in FIG. 1bshows ρ versus V_(g) at 295 K after the device was heated to 375 K.Here, the CNP has shifted to negative V_(g), a nearly 50 V shift, whileρ_(max) was seen to change by only about 12% of its initial value. Theshift of the CNP to negative V_(g) indicates that without the presenceof water on the surface, the hydrogenated graphene film has changed fromp-type to n-type. Raman spectra collected after heating show that nomeasurable change occurs in the D/G ratio due to heating to 375 K, andwe conclude that the change in hydrogen content after heating isnegligible.

FIG. 1(c) shows ρ versus V_(g) for device HG-Hb after having left thehydrogenated sample under vacuum for at least 24 hours before anyelectrical measurements were carried out. Consistent with the slowremoval of adsorbed water in vacuum, the CNP is seen to shift tonegative V_(g) values even without heating. The data in FIG. 1(c)mirrors the symmetry seen in FIG. 1b for hydrogenated graphene withadsorbed water and definitively shows that without water adsorbed to thefilm's surface, the hydrogenated graphene material is increasinglyn-type for increasing levels of hydrogenation.

It is surprising that although the hydrogenated film displays n-typebehavior, the incorporation of water—a known electron donor—physisorbedon the surface results in a p-type material. Our DFT model indicatesthat this behavior is due to changes in the materials work function(WF). While graphene's WF is very close to that of the substrate(thermal oxide on silicon), evidenced by its CNP being close to 0 V_(g),the WF of the hydrogenated material is higher, leading to anelectron-enrichment of the film that accounts for our observations(n-type). Furthermore, we observe that our model predicts that thephysisorption of water leads to a significant lowering of the WF wellbelow that of graphene (and thus that of the substrate), FIG. 1(d). Afilm WF lower than that of the substrate's results in an electrondepletion (electrons moving from the film to the substrate), resultingin a p-type material. The different type of majority carriers is thusaccounted for by the WF's of the different materials relative to thesubstrate, and such a striking change in majority carrier should only beobserved in substrates whose WF is close to that of graphene.

The WF of graphene has been measured and is almost identical to that ofgraphite, ˜4.6 eV, very close to the reported value for a >100 nmthermal oxide layer on n-type silicon, which explains why our exfoliatedgraphene on a SiO₂ substrate always displays a CNP close to 0 V_(g). Asstated above, we performed DFT calculations to assess the effect ofhydrogenation and subsequent water adsorption on the WF of graphene. Forthese purposes, we use a cluster model consisting of a coronene moleculeusing a triple-zeta Gaussian basis [6-311 G(d,p)] and the wB97XDfunctional as implemented in the Gaussian 09 software suite. To accountfor the effect of the positively charged substrate, we included anelectric field in the direction perpendicular to the molecular plane.The relative changes on the WF were estimated by using the approximationWF=−(e_(HOMO)+e_(LUMO))/2. The WF for coronene is estimated to be 3.74eV, and although this is about 0.9 eV lower than the experimental valuefor graphene, we are interested in its changes due to hydrogenation andsubsequent water adsorption. The hydrogenated coronene displays a WF˜0.1 eV higher than that of coronene, while the absorption of waterresults in a lowering relative to coronene of ˜0.15 to 0.20 eV,consistent with the discussion above. We emphasize the importance ofaccounting for the substrate effect through an electric field, as whenno field is present the WF's for the hydrogenated graphene material withand without water are both approximately ˜0.06 eV lower than that ofcoronene.

We further study the electronic properties of the hydrogenated graphenewith several additional devices including a square geometry (referred tohere as device G-Sq) shown in FIG. 1a , and a cross geometry with armdimensions of 1.25 μm by 500 nm (length and width respectively, andreferred to as device G-X2), which all demonstrate the same effect. Theblack traces in FIGS. 2(c) and 3(b) illustrate ρ versus V_(g) for thegraphene found at 295 K. The graphene was heated to 375 K and the CNPwas determined to remain fixed in V_(g) at 295 K with no appreciablechange in the ρ_(max) value as seen in FIG. 2(b). The graphene 295 Kmobility μ_(h) (p-type, hole conduction) for the G-Sq device wasmeasured at 0 V_(g) and found to be 8,300 cm²/Vs, while the 295 Ktemperature carrier density n_(h) was found to be 9.4×10¹¹ cm⁻².

The two samples were hydrogenated to D/G ratios of 3.9 (HG-Sq) and 2.1(HG-X2) and were evacuated in the cryostat for 17 hours before anyelectrical measurements were carried out. The CNP for both hydrogenatedsamples shifted ˜20V to the left in V_(g) from the CNP of the pristinegraphene even without heating, as can be seen in FIG. 2c (HG-Sq) andFIG. 3(b) (HG-X2). After the CNP had shifted the mobility and carrierdensity were measured for the HG-Sq sample at 0 V_(g): μ_(e) (n-type,electron conduction)=307 cm²/Vs and n_(e)=7.4×10¹¹ cm⁻².

The samples were heated to 375 K though no appreciable change wasobserved in the location of the CNP. Thus we conclude that the majorityof the water on the surface was desorbed during the extended time invacuum. Together with the small changes in D/G ratios before and afterthe measurements and heating cycles, these observations indicate thatthe shift in carrier type is exclusively due to physisorption/desorptionof water on the surface and not through a chemical reaction.Reversibility of the carrier type upon exposure of the film toatmospheric water further confirms this hypothesis as the CNP was seento shift back to the right after exposing the HG-Sq sample to deionizedwater and subsequently measuring ρ versus V_(g).

We investigated ρ versus T in the 50 K to 375 K range for the twodifferent D/G ratios, and at various V_(g). FIGS. 2(c) and 3(b) showthat ρ for each hydrogenated device changes sharply about the CNP withdecreasing T, but that this change is not as severe when V_(g) is sweptfurther away from the CNP. This semi-insulating behavior fits well tothe two dimensional Variable Range Hopping (VRH) theory described byequation (3), which is demonstrated in FIG. 2d for the device with a D/Gratio of 3.9 (HG-Sq). The inset in FIG. 2(d) plots the characteristicexponents T_(o), found from the fit to the VRH theory, as a function ofV_(g), and is well fitted by a Lorentzian model. The values for ρ_(o)found from the VRH fit had a mean value of 5,982 Ω/sqr and deviated fromthis value by no more than 20%.

$\begin{matrix}{\rho = {\rho_{o}e^{{(\frac{T_{o}}{T})}^{1/3}}}} & (3)\end{matrix}$

Changes in ρ as a function of T (Δρ/ΔT) increased with increasing levelsof hydrogenation and suggest the opening of a bandgap, Δ∈. An estimateof Δ∈ is deduced from the T dependence of the intrinsic conductivityτ(1/ρ), which for a homogeneous semiconductor varies exponentially asshown in equation (4). FIG. 4(a) plots the logarithmic behavior σ versus½K_(B)T where the slope of the line should be proportional to thebandgap.

$\begin{matrix}{\sigma \propto e^{\frac{{- \Delta}\; ɛ}{2{kT}}}} & (4)\end{matrix}$

We find for each hydrogenated sample that the maximum bandgap occurs atthe CNP and decreases with V_(g) away from the CNP, as seen in FIG.4(b). Furthermore, based upon the data in FIG. 4(b) we conclude that alarger D/G ratio in the Raman spectra, and therefore a higherconcentration of hydrogen atoms adsorbed to the graphene, will lead to alarger bandgap opening at the CNP. Although a maximum Δ∈ of ˜50 meV ismeasured in our lightly hydrogenated devices, we note that this is not alimit as the calculated bandgap for hydrogenated graphene increases upto 5.4 eV as you increase the hydrogen coverage. We determine that theon/off ratio (the ratio of the low vs. high resistance points in the Rvs. V_(g) plots) remains virtually fixed for graphene, but increaseswith decreasing temperature for hydrogenated graphene. For the datashown in FIG. 2(c) the ratios of the two materials are approximatelyequivalent at 50K, with the hydrogenated graphene on/off ratioincreasing further with decreasing temperature. The fact that the on/offratio for hydrogenated graphene is still low at room temperature couldbe the result of the creation of a small bandgap (˜50 meV) and/ortransport channels that are manifested within the bandgap.

DFT calculations have been carried out within the plane-wavepseudo-potential approximation and indicate that for free standing films(graphene, partially hydrogenated graphene, and partially hydrogenatedgraphene with water without accounting for substrate effects) there is anegligible bandgap, which has been further confirmed by orbital-basedDFT calculations that yield the same conclusion. Also, the zero-bandgapin graphene has been shown to be very robust toward deformation andstress, so that the stress induced by the substrate (which should behigher upon hydrogenation) does not account for the bandgap either. Weconclude that the observed bandgap can be attributed to one or acombination of the following factors: (a) long-range disorder, (b) thelarge electric fields that a positively charged substrate such asthermal oxide would exert on the films, and (c) the electron densitydepletion/increase induced by the difference in WF between substrate andfilm.

The ability to control the majority carrier type while introducing abandgap makes hydrogenated graphene a promising method for nanocircuitdesign (e.g. p-n junctions) in a graphene-based system. Tunability ofthe carrier type via surface doping removes the requirement for multiplegate electrodes for independent carrier type control, avoiding the needfor high quality dielectrics that are difficult to achieve on grapheneand are susceptible to leakage currents. Our work also demonstrates howsurface adsorbates can affect the electrical properties of hydrogenatedgraphene, properties that would otherwise be negligible in bulkmaterials.

EXAMPLES

The graphene devices listed in Table 1 were fabricated by mechanicalexfoliation of HOPG on a SiO₂ (275 nm)/Si (n-type arsenic doped)substrate, followed by the deposition of Cr (10 nm)/Au (50 nm) contactelectrodes. For devices that required additional geometrical patterninga low power O₂ plasma treatment was used to etch the film into thedesired shape. Hydrogenation of the graphene was performed according tounder the following conditions: 15-30 W, 1.5 Torr H₂, 100 sccm, 32° C.,for 15-30 seconds. The hydrogenation conditions (time and power) varieddepending upon the desired level of hydrogenation. Raman spectroscopywas used to determine the relative defect densities in the films by aratio of the G-mode intensity (1588 cm⁻¹, E_(2g) phonon mode) and theD-mode intensity (1345 cm⁻¹, appearing due to symmetry breaking atdefect sites). The D/G ratio is related to the defect-free domain sizeof graphitic materials, in this case caused by hydrogen addition to thegraphene sheet. With the reactor conditions stated, the resulting D/Gratios demonstrated saturation for each power/time and were repeatablefor all devices. Raman spectra were collected using a RenishawMicroRaman Spectrometer with a 514 nm laser excitation.

TABLE 1 Properties of graphene (G), hydrogenated graphene (HG), andhydrogenated graphene with water adsorbates (HG + W) for the fourdifferent samples X1, Sq, X2, and Hb. Sample D/G ratio CNP (V) Carriertype ρ_(max) (at room T) G-X1 0 1 p 4,100 Ω/sqr HG-X1 + W 0.67 6 p 6,800Ω/sqr 1.68 21 p 16,100 Ω/sqr 2.94 34 p 43,300 Ω/sqr HG-X1 2.94 −13 n51,000 Ω/sqr G-Sq 0 10 P 3,500 Ω/sqr HG-Sq 3.9 −11 n 33,300 Ω/sqr G-X2 030 p 4,200 Ω/sqr HG-X2 2.1 9 p 15,500 Ω/sqr G-Hb 0 6 p 1,100 Ω HG-Hb1.24 −6.4 n 2,500 Ω 2.7 −10.3 n 4,600 Ω

Electronic transport measurements were carried out in a cryogenic probestation, using AC lock-in techniques at a frequency of 13.7 Hz. Afour-wire configuration is used in all of our measurements, as seen inFIG. 1a . An excitation current of 10 nA was used for the hydrogenateddevices (I=31.6 nA for pristine graphene) and the voltage drop acrossthe device was monitored and kept below K_(B) T/e to prevent chargecarrier heating, where K_(B) is the Boltzmann constant and e thefundamental unit of electric charge. ρ for each device is found usingequation (1), where I_(ab) is the source current along one edge of thesample and V_(cd) the voltage drop measured across the opposite edge(except for the Hall bar device G-Hb, in which case ρ is calculated fromthe resistance R via ρ=RW/L, where W and L are sample width and length,respectively).

$\begin{matrix}{\rho = {{\frac{\pi}{\ln\; 2} \cdot \frac{1}{4}}\left( {\frac{V_{43}}{I_{12}} + \frac{V_{14}}{I_{23}} + \frac{V_{21}}{I_{34}} + \frac{V_{32}}{I_{41}}} \right)}} & (1)\end{matrix}$The Hall mobility of the charge carriers (μ) is calculated usingequation (2), where the Hall resistance (R_(H)) and ρ were measured in afield of 220 mT. The values for the hydrogenated graphene were measuredafter the CNP had stopped shifting to the left in V_(g), which undervacuum suggests that the majority of the water had been desorbed fromthe film's surface.

$\begin{matrix}{\mu = \frac{R_{H}}{\rho}} & (2)\end{matrix}$

Tunability of the majority carrier type via surface doping removes therequirement for multiple gate electrodes for independent carrier typecontrol, avoiding the need for high quality dielectrics that aredifficult to grow on graphene and are susceptible to leakage currents.The additional capability of being able to introduce a bandgap ingraphene materials, in parallel to controlling the majority carriertype, makes hydrogenated graphene a promising method for nanocircuitdesign in a graphene-based system.

The method disclosed herein is superior to the prior art, solveslong-standing problems, and provides several new advantages. Forexample, bilayer graphene and top-gated single layer graphene structuresprovide only a small bandgap or no bandgap. Fluorinated graphene mayhave bandgaps, however, it is not a tunable process. Here, the methodprovides a larger bandgap that is tunable and reversible, which offersunique benefits.

The above examples are merely illustrative of several possibleembodiments of various aspects of the present disclosure, whereinequivalent alterations and/or modifications will occur to others skilledin the art upon reading and understanding this specification and theannexed drawings. In addition, although a particular feature of thedisclosure may have been illustrated and/or described with respect toonly one of several implementations, such feature may be combined withone or more other features of the other implementations as may bedesired and advantageous for any given or particular application. Also,to the extent that the terms “including”, “includes”, “having”, “has”,“with”, or variants thereof are used in the detailed description and/orin the claims, such terms are intended to be inclusive in a mannersimilar to the term “comprising”.

What is claimed is:
 1. A graphene compound made from the method ofpreparing graphene flakes or chemical vapor deposition grown graphenefilms on a SiO₂/Si substrate; exposing the graphene flakes or thechemical vapor deposition grown graphene film to hydrogen plasma;performing hydrogenation of the graphene; wherein the hydrogenatedgraphene has a majority carrier type; creating a bandgap from thehydrogenation of the graphene; applying an electric field to thehydrogenated graphene; tuning the bandgap; controlling the majoritycarrier type via surface adsorbates; attaching by physisorption thesurface adsorbates to the hydrogenated graphene; converting the majoritycarrier type from electrons to holes using the surface adsorbates;converting the majority carrier type from n-type to p-type; removing thesurface adsorbates; preserving the hydrogenated graphene band structure;and converting the majority carrier type from p-type to n-type.
 2. Agraphene compound comprising hydrogenated graphene with a bandgap andexhibiting n-type nature wherein the bandgap is tunable with an electricfield effect and wherein the hydrogenated graphene has a majoritycarrier type and a bandgap created from the hydrogenation of thegraphene and wherein an electric field applied to the hydrogenatedgraphene tunes the bandgap; wherein the majority carrier type iscontrolled via surface adsorbates; wherein the bandgap is higher forhigher hydrogen to carbon H/C ratios; and wherein the bandgap has amaximum value at the charge neutrality point (CNP).
 3. A graphenecompound comprising hydrogenated graphene with a bandgap and exhibitingn-type nature further including surface adsorbates physisorbed to thehydrogenated graphene wherein the surface adsorbates are water andresults in a p-type material wherein the hydrogenated graphene is formedby the process of preparing exfoliated graphene flakes or a chemicalvapor deposition (CVD) grown graphene film on a SiO₂/Si substrate,depositing contact electrodes on the exfoliated graphene flakes or theCVD grown graphene films, exposing the exfoliated graphene flakes or thechemical vapor deposition grown graphene film to hydrogen plasma,hydrogenating the exfoliated graphene flakes or the CVD grown graphenefilms, creating a bandgap from the step of hydrogenating the graphene,applying an electric field to the hydrogenated graphene, and tuning thebandgap; wherein the contact electrodes are Cr/Au contact electrodes andthe hydrogenating is performed with 15-30 W, 1.5 Torr H₂, 100 sccm H₂,32 C, for 15-30 seconds.